For example, as disclosed in JP-A-2005-92490, a wireless remote sensing system using a surface acoustic wave (SAW) sensor is known. A system disclosed in JP-A-2005-92490 includes a SAW sensor and a sensing apparatus. In the sensing apparatus, a mixer mixes a carrier wave, having a predetermined frequency, outputted from an oscillator with a pulse signal outputted from a pulse generator by a mixer. Thus, the carrier wave is pulse-modulated so that a transmission signal can be generated. The transmission signal is amplified by an amplifier and then transmitted to the SAW sensor. When a reception signal is received from the SAW sensor, a delay time from when the transmission signal is transmitted to when the reception signal is received is analyzed. Thus, a physical quantity such as distortion or temperature is detected by a contactless method.
FIGS. 14 and 15 illustrate a detection principle of a conventional wireless remote sensing system 1. The wireless remote sensing system 1 includes a SAW sensor 2 and a sensing apparatus 3. The SAW sensor 2 includes a SAW reflector 7. The SAW reflector 7 has a piezoelectric substrate 4, a driving electrode 5 formed on the substrate 4, and a reflector electrode 6 formed on the substrate 4. The sensing apparatus 3 includes an oscillator 8, a first switch 9, a first amplifier 10, a second switch 11, a second amplifier 12, a mixer 13, and a low-pass filter 14.
As shown in FIG. 14, when the sensing apparatus 3 transmits a transmission signal to the SAW sensor 2, a movable contact 9a and a transmission contact 9b of the first switch 9 are connected so that the first switch 9 can go into a transmission state, and a movable contact 11a and an input contact 11b of the second switch 11 are connected so that the second switch 11 can go into an input state. At this time, a carrier wave, having a predetermined frequency, outputted from the oscillator 8 is pulse modulated by the first switch 9 so that a transmission signal can be generated. The transmission signal is amplified by the first amplifier 10 and transmitted through the second switch 11 to the SAW sensor 2. In the SAW sensor 2, the transmission signal is converted by the driving electrode 5 into a surface acoustic wave, and the surface acoustic wave propagates to the reflector electrode 6 by passing on the substrate 4. Then, the surface acoustic wave is reflected by the reflector electrode 6 and propagates to the driving electrode 5 by passing on the substrate 4. The surface acoustic wave is converted by the driving electrode 5 into a reception signal.
As shown in FIG. 15, when the sensing apparatus 3 receives the reception signal from the SAW sensor 2, the movable contact 9a and a reception contact 9c of the first switch 9 are connected so that the first switch 9 can go into a reception state, and the movable contact 11a and an output contact 11c of the second switch 11 are connected so that the second switch 11 can go into an output state. At this time, the reception signal transmitted from the SAW sensor 2 is amplified by the second amplifier 12 and inputted to the mixer 13 through the second switch 11. Further, the carrier wave outputted from the oscillator 8 is inputted as a local signal to the mixer 13. Thus, in the mixer 13, the reception signal and the local signal are mixed so that a synchronous detection signal can be generated. A high frequency component of the synchronous detection signal is removed by the low-pass filter 14 so that the synchronous detection signal can be outputted as a direct current (DC) signal from an output terminal 15 to external circuitry.
For example, the integration density of the sensing apparatus 3 can be increased by implementing the second switch 11 and the mixer 13 on a one chip of a silicon substrate 16, which is a semiconductor substrate. In this case, however, when the sensing apparatus 3 receives the reception signal from the SAW sensor 2, the carrier wave outputted from the oscillator 8 may pass on the silicon substrate 16 and be inputted as a leak signal to the second switch 11. If the leak signal is inputted to the second switch 11, it is difficult for the sensing apparatus 3 to accurately receive the reception signal from the SAW sensor 2. As a result, a sensing accuracy of the sensing apparatus 3 may be degraded.